A significant number of patients return with persistent symptoms following surgical release of the posterior interosseous nerve for radial tunnel syndrome. The aim of this study was to attempt to explain this fact in anatomical terms by defining the anatomy of the posterior interosseous nerve and its branches in relation to the supinator muscle and arcade of Frohse. Using standard dissection tools 20 preserved cadaveric upper limbs were dissected. The radial nerve and all its branches within the radial tunnel were exposed and a digital calliper was used to measure distances. The bifurcation of the radial nerve to posterior interosseous nerve and superficial sensory branch occurred at a median distance of 4.35mm proximal to the elbow joint-line. The bifurcation was proximal to the joint-line in 11 cases, at the level of the joint-line in one case and distal in eight cases. There was a range of 0–5 branches to the supinator originating proximal to the entry point of the posterior interosseous nerve under the arcade of Frohse at a median distance of 10.27mm (medial branches) or 11.11mm (lateral branches) distal to the elbow join-line. These branches either passed under the arcade of Frohse or entered through the proximal edge of the superficial belly of the supinator. In 10 limbs there was a variable number of branches to the supinator originating under its superficial belly and in five limbs multiple perforating posterior interosseous nerve branches within the muscle were identified. This variation in anatomy we believe may explain the persistence of symptoms following surgical release of the posterior interosseous nerve for radial tunnel syndrome and suggests that careful exploration of all the nerve branches during surgical decompression should be routinely performed.